All commercial consumer motor casing are required to fail in a controlled manner should a massive overpressurization occur due to a mechanical grain failure. (See NFPA 1125) Since the airspace above and below the rocket is clear before launch, the forward and aft closures are designed to fail at pressures that are 2 to 3 time higher than the expected maximum chamber pressure, but 2 to 3 times lower than the case rupture pressure. This spits out the nozzle and/or the forward closure and the fuel grains, preventing the generation of shrapnel in a CATO. All commercial consumer casing must be made from 6061T6, a strong aluminum alloy that yields well below it tensile strength so it will not fragment into shrapnel.
Almost anyone can make a decent C-ring groove on a lathe, whereas it take a reasonably good machinist to cut quality threads. If you setup in a lathe, you could easily cut 2 internal C-ring grooves in a minute. It will take several minutes to cut in internal thread unless you have a CNC machine. It's simply cheaper for low volume operations to go with C-rings.
Also lathe work is much faster and cheaper than drilling and tapping. Threading fine threads in graphite is a job for the pros and certainly 4-40 threads would pull out unless you had an ungodly number of them. And in the end, you don't need to buy the taps or the screws with C-rings.
Machining nozzles from graphite is really simple when it's all lathe work. Once you have the OD finished and the graphite cut to length, it should take only 10 to 15 minutes to make a 54 mm nozzle if you are set up on a lathe to do it: Drill the throat in 2 or 3 steps, put a slight dish on the chamber side, a 15 degree half angle cone on the exit, and pop in 2 o-ring grooves on the od.
The only disadvantage with C-rings is the need to have a good set of snap ring pliers and the need to being careful when inserting or removing them. A slip of the compressed ring could result in personal injure, especially to the eye, if safety glasses and precautions are not taken.
In practice, motors with snap rings go together a whole lot quicker than with threaded fitting and with fewer parts. The Kosden/Animal Works approach is simpler for the Ex guy making his own casings. A cheap source of C-ringed hardware is the purple stuff from
https://www.aeroconsystems.com/motors/diy.htm
Bob Krech